Rechargeable battery packs have become a defacto standard for use with portable consumer electronic devices such as computers, cellular telephones, cordless phones, power tools, and the like. Consequently, a significant amount of engineering effort focuses on the development of new and improved methods and systems for charging battery packs.
One goal of these engineering efforts is to develop charging algorithms that alleviate the memory cycle effect common to many battery technology types such as AqZn, NiZn and Lithium Ion. The memory cycle effect is the tendency for a battery cell to remember the maximum voltage potential obtained during previous charge cycles and then to limit future charges to this value. Thus, if a battery cell is undercharged (or partially charged) for several charge cycles, the battery cell will develop a resistance to being fully charged during future charge cycles. This tendency results in decreasing the charge life (time between charges) of a battery cell and hence, decreases the charge life of a battery pack that contains that battery cell. In addition, the performance of battery cells tends to diminish with the number of charge cycles the battery cells experience. Because a decrease in the charge life of a battery cell will require the battery pack to be charged more frequently, the overall life of the battery pack will also be reduced.
Another goal of these engineering efforts is to develop charging algorithms that quickly and completely charge a multiple-cell battery pack without overcharging the individual battery cells. Once a battery cell has been fully charged, a continuous source of current may damage the structure of the battery cell. Thus, similar to undercharging a battery cell, overcharging a battery cell may also significantly affect the life of a battery cell and the battery pack.
Thus, overcharging and/or undercharging cells within a battery pack may result in decreasing the overall life span of a battery pack. Furthermore, a significant portion of the cost of portable electronic devices can be attributed to the cost of the battery pack. Therefore, a need exists to maximize the life span of rechargeable battery packs.
Generally, battery packs for portable electronic devices include several battery cells connected in series. Ideally, each of the battery cells within a battery pack will have similar charging, discharging and efficiency characteristics. However, this ideal scenario is not easily achieved. In order to build these ideal battery packs, manufacturers must expend considerable time and expense to measure and characterize several battery cells and then group together the battery cells with similar characteristics. This results in a dramatic increase in the production cost of the battery packs. Thus, more often than not, a battery pack contains multiple battery cells with each battery cell having different charging, discharging and efficiency characteristics. This tendency results in exaggerating the previously described problems due to overcharging and undercharging of the battery cells. For instance, fully charging one battery cell in a battery pack may result in overcharging one or more of the other battery cells in the battery pack. Likewise, ending a charge cycle when only one battery cell is fully charged may result in undercharging one or more of the other battery cells in the battery pack. Therefore, there is a need for a system to provide a balanced charging cycle that accommodates multiple battery cells having varying charging, discharging and efficiency characteristics.
Several techniques have been developed to address the problems involved in charging multiple-cell battery packs. One of these techniques is described in U.S. Pat. No. 5,283,512 to Stadnick et al. Stadnick describes a system for charging multiple batteries or battery cells connected in series. This system charges each of the battery cells at a full current rate until one battery cell reaches a maximum threshold voltage (i.e., is fully charged). The full current rate is then turned off and a low current rate, referred to in the art as a trickle charge, is started. As each battery cell becomes fully charged during the trickle charge, the battery cell is removed from the charge path. This is accomplished by shunting the current of the trickle charge around the fully charged battery cell.
The system described in Stadnick has at least two disadvantages. First, in order to avoid damaging the battery cells, the trickle charge must be significantly less than the full current rate. This requires a shunting device that can dissipate the difference between the full current rate and the trickle charge current rate. Additionally, dissipating this amount of current is wasteful and results in generating a significant amount of heat. This heat must be dissipated by the use of heat sinks or the like. A shunting device that can handle this amount of current is typically expensive compared to the other components in the charging circuitry. Second, the system described in Stadnick can result in significantly lengthening the charge cycle time. For example, if one battery cell in the battery pack reaches the maximum threshold voltage much earlier than the other battery cells, the amount of time required to charge the remaining battery cells under a trickle charge may be significant. Therefore, there exists a need for a charging system and method that balances the charge of a multiple-cell battery pack without increasing the cost of the battery pack by requiring a shunting device to dissipate a large amount of current. Furthermore, there exist a need for a balanced charging system that does not significantly increase the charge cycle time.
A second technique for charging multiple-cell battery packs is described in U.S. Pat. No. 5,504,415 to Podrazhansky et al. Podrazhansky describes a method of charging multiple batteries or battery cells connected in series. This system utilizes a thermistor to detect when a battery cell is fully charged or is approaching a fully charged state. Generally, by monitoring the thermal characteristics of a battery cell, a determination of when the battery cell has been fully charged can be made. For instance, the temperature may change at one rate while the battery cell is charging and then at a different rate once the battery cell has been fully charged. Thus, as a particular battery or battery cell approaches the fully charged state, the charging current is shunted around the battery or battery cell to prevent overcharging.
The system described in Podrazhansky has at least two disadvantages. First, the use of a thermistor for each battery cell in the battery pack increases the cost of manufacturing the battery pack. Also, the thermistors must be carefully placed so as not to be affected by the temperature of surrounding cells or by heat generated by components in the attached equipment. For instance, in a cellular mobile telephone, the design process must take into consideration the location of heat generating components, such as a power amplifier, in order to prevent degrading the accuracy of the thermistors. Second, Podrazhansky teaches completely shunting the current around a battery or cell that approaches the fully charged state. Similar to Stadnick, this results in increasing the cost of the shunting device. Therefore, there is a need for a balanced charging system that does not impact the design and cost of attached devices or the battery pack.
A third technique for charging multiple-cell battery packs is described in U.S. Pat. No. 5,498,950 to Ouwerkerk. Ouwerkerk describes a method to balance the charge of multiple batteries or battery cells connected in series. This method operates by sequentially measuring the voltage level of each battery or battery cell. The batteries or battery cells having the lowest voltage, relative to the other batteries or battery cells, are then charged. After this charge cycle, the batteries or battery cells are again sequentially measured to determine which batteries or battery cells will be charged in the next charge cycle.
The system described in Ouwerkerk has at least one disadvantage. For each charge cycle, only a portion of the batteries or battery cells are selected to be charged. Thus, multiple charge cycles must be performed in order to fully change an entire battery pack. This results in significantly increasing the charge cycle time for the battery pack.
Thus, there exists a need for a balanced charging system that does not significantly increase the charge cycle time for a multiple-cell battery pack. There also exists a need for a balanced charging system that does not require expensive shunting devices for diverting or dissipating large amounts of charging current. There also exists a need for a balanced charging system that does not impact the design and cost of the attached equipment. Furthermore, there exists a need for a balanced charging system that, on the average, charges each cell of a multiple-cell battery pack to its maximum value and thereby does not adversely affect the life cycle of the battery pack.